1. Field of the Invention
The present invention relates to a method of correcting shading in an image reading system of an exposure scanning type, etc., and an apparatus therefor.
2. Description of the Prior Art
In the image reading system, the nonuniformity of the quantity of light occurs on the light-receiving plane of an image pickup element due to the characteristics of a light source, a focusing lens, etc., while picture elements of the image pickup element are nonuniform in sensitivity, and therefore shading occurs in read image signals.
In order to correct this shading, a method using an A/D converter has been proposed heretofore. When the shading affects image signals of one line containing the maximum brightness Io of an image that is obtained by picking up a reference density board (e.g. a while board), characteristics as shown in FIG. 6 are revealed. If the coefficient showing the shading effect at coordinates (x, y) in this figure is denoted by S (x, y), an image signal f (x, y) is expressed by EQU f (x, y)=Io.multidot.S (x, y) . . . (1).
If a signal obtained by sampling f (x, y) is denoted by f (m, n) (m,n.epsilon.Z, Z.DELTA.{0,.+-.1,.+-.2, . . . }) in consideration of the case when said image signal is given to an A/D converter of a parallel comparison type to be sampled and quantized, an output D (m, n) in the A/D converter is represented by the following equation. ##EQU1## where K denotes a scaling coefficient, and V.sub.ref a reference voltage of the A/D converter.
Next, when an image not subjected to the shading effect is denoted by fo (m, n) and a coefficient obtained by sampling S (x, y) is denoted by S (m, n), EQU f (m, n)=fo (m, n).multidot.S (m, n) . . . (3) is established from the relation of the equation (1), and therefore an equation ##EQU2## can be established from the equations (2) and (3). An image signal cleared of shading by correction in real time can be obtained by varying the reference voltage V.sub.ref so that it be in proportion (the constant is denoted by G) to S (m, n). That is, EQU V.sub.ref =G.multidot.S (m, n) . . . (5).
From the equations (4) and (5), D (m, n) is determined as ##EQU3## Thus, the output D (m, n) of the A/D converter turns to be an image signal being independent of the shading effect, i.e. a signal cleared of shading by correction.
Accordingly, a method has been proposed heretofore wherein: a reference white called a reference density board is picked up before an image is read, a pickup signal thus obtained is converted into a digital signal by an A/D converter 1 and then stored in a one-line memory 3 through a tri-state buffer 2 as shown in FIG. 7, the signal stored therein is read out as a shading correction signal at the same timing with that of one-line scanning by a pickup element on the occasion of subsequent reading of the image and is passed through a tri-state buffer 4 and converted into an analog signal by a D/A converter 5, and a conversion output therefrom is applied as the reference voltage V.sub.ref on the A/D converter 1 so that the shading correction be executed simultaneously with the conversion in the A/D converter 1.
The reference voltage V.sub.ref of the A/D converter 1 applied thereon When the shading correction signal is read is supplied as a voltage obtained by converting the full-scale entire bit high level data from a tri-state buffer 6 into an analog signal by the D/A converter 5. Numeral 7 denotes an inverter for selecting either the buffer 4 or 6, and mark A a reference voltage switching signal. FIG. 6 shows the construction of the A/D converter 1 of a parallel comparison structure employed therein. Mark 1a denotes a split resistor group, 1b a comparator group, 1c an encoder, and 1d a latch. FIG. 8 is a timing cart of the operation of a circuit shown in FIG. 9.
In the shading correcting method shown in FIG. 7, however, a range of conversion is 0v substantially, and a signal to be cleared of shading by correction (i.e. an input signal to the A/D converter 1) and a shading correction signal (the reference voltage V.sub.ref) are to have the same voltage, since the data of the shading correction signal stored in the one-line memory 3 are applied, without any modification, as the reference voltage V.sub.ref of the A/D converter 1 at the time of correction, particularly in an A/D conversion operation out of the effective period of an image. Moreover, considerable noise from other circuits or the like is mixed in these signals, and thereby a change in voltage is caused in both the signal to be cleared of shading by correction and the shading correction signal.
The A/D converter 1 conducts determination, therefore, on the basis of the respective changes in voltage of the signal to be cleared of shading by correction and the shading correction signal, but the result is that the output thereof is set to be either the maximum value (all bits are at high level) or the minimum value (all bits are at low level), since the range of conversion is 0v substantially as stated above.
When this shift of the output to the maximum or minimum value occurs in a relatively short cycle by the effect of noise or the like, a number of comparators, output circuits, etc. inside the A/D converter repeat ON/OFF operations almost at the same time, and consequently a large variation in current occurs in the A/D converter.
This large variation in current involves a relatively high frequency, and it is a variation not existing in a signal waveform. Therefore it has a large possibility of causing an effect as the noise of the signal to be cleared of shading by correction. Since a relatively large amount of current flows through a source of generation of the noise, in addition, there is a large possibility that it mixes in a power source having small impedance and a grand line as the noise having a larger amplitude than in an ordinary case.
The noise caused by the values of the signal to be cleared of shading by correction and of the shading correction signal of this A/D converter mixes in a black level which is to operate as a reference level for pickup signals, as shown in FIG. 9, and this causes a large fluctuation of the black level, which has an extremely adverse effect on the formation of an image.
According to the conventional method, as described above, the deterioration of the quality of the image is large considerably due to the generation of the noise caused by the voltage values of the signal to be cleared of shading by correction and the shading correction signal which are applied on the A/D converter out of the effective period of the image.